Science Questions

What would the planets do without the sun?

Tue, 17th May 2016

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Question

Munyaradzi Tobaiwa asked:

If the sun disappeared, what would happen to the planets?

Answer

We asked Astronomer Gerry Gilmore what he thought of this one...

Gerry - The planets would keep moving exactly as they are, instantaneously. The HD 69830 Solar SystemPeople imagine that planets naturally move in circles, but they donít. Everything naturally moves in a straight line. A planet moves a little bit in a straight line and it falls a little bit towards the sun, and it moves a little bit more and falls towardsÖ

Chris - And thatís gravity tugging it inwards?

Gerry - Yes, thatís right. Itís falling the whole time but itís also moving in a straight line the whole time. So, if the gravity were simply to stop, the planets would just continue in the straight line they are currently moving in, thatís the tangent as we call it, to their present orbit, and so they would just carry on moving away. The underlying presumption here is when all this happens is kind of interesting because people imagine that there is some sort of and absolute underlying time, a newtonian time, and that therefore all the planets will head off instantaneously at the same time. But, actually, time is different on the different planets so it takes time for gravity, and light, and everything else to come from the sun to us.

Chris - Ah right, so gravity isnít there instantly. What youíre saying is if the sun did evaporate all of a sudden in an instant, then we wouldnít know for a certain amount of timeÖ

Gerry - Exactly. If the sun suddenly went dark, then the sky would stay bright for as long as it took the light to get to us.

Chris - So you're saying that gravity propagates at the speed of light?

Gerry - Yes and so does time, actually. Everything goes at the speed of light. Both time and gravity and light all travel at the speed of light.

Chris - Why do we think that gravity propagates at the speed of light then?

Gerry - We know it does actually, it was measured early this year.

Chris - With the gravitational wave stuff?

Gerry - With the gravitational wave burst, yes.

Chris - And how did they measure that then?

Gerry - There were two detectors separated by a finite distance and you could measure the timeÖ

Chris - The gravity wave arrives at one and they know how far away it is to the other one?

Gerry - Yes, and a few milliseconds later the other one which was a few thousand kilometers away and thatís fundamental to Einsteinís general theory of relativity, was a prediction from almost immediately after the theory was developed, and it was completely different than the way it works in newtonian gravity where there is this absolute time sitting there in the background that things just live in, whereas we now know that thatís not correct.

Chris - So to summarise for the benefit of Munyuradzee who sent this question in. Were the sun or a star with some planets going round it suddenly to disappear itís gravitational influence would be removed, and the time it took before those planets ceased to feel the gravity from that star would be however long it takes light from that star to reach that planet.

Gerry - Absolutely. So if youíre outside watching it you would see the planets popping off in their straight line orbits but, because everything happening in the time it takes the signal to get to you, youíd see it all just vanishing off very nicely. Itís only if you were one one of those planets youíd think it was different.

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They would fly away at a tangent to their orbits. alancalverd, Fri, 22nd Apr 2016

Until they encountered a deeper well of gravity, then they would either orbit or collide or perhaps both.

Would the Earth and the Moon remain paired up???  It would make a bit of difference where the moon was in orbital relation to the Earth when the gravity wave passes. JoeBrown, Sat, 23rd Apr 2016

To expand on Alan's point, it would depend upon where the planets were in their orbits in relation to one another. There could be cases in which two tangents intercept in which case the planets would collide. In other cases two or more planets may end up moving away in unison having entered into a system of complex orbits. jeffreyH, Sat, 23rd Apr 2016

Something occurred to me after posting the above. If we have a function c(x) that describes a circular orbit and a function p(x) that describes the orbital path around a galaxy the function p(c(x)) will describe increasing orbital speed with increasing distance from the galactic centre. Therefore time dilation will increase with radial distance. I haven't considered the implications yet. jeffreyH, Sat, 23rd Apr 2016


A planet approaching another star from interstellar space will be traveling with greater velocity than the escape velocity of that star.

So if there are no other planets around the other star, it will follow a hyperbolic or parabolic orbit, and just pass once; you wouldn't call this an orbit.

However, if the other star has planets, it could:

Collide, as you say: This is unlikely in a single pass

Interact gravitationally: Transferring some of its momentum to another planet could slow down the newcomer enough to enter orbit around the star. After which it it will continue to disturb the orbits of the existing planets, possibly triggering the planetary orbits to become chaotic. It could well collide itself, or cause other planets to collide.



The way astronomers applied Kepler, Newton and Einstein, the orbital speed should decrease the farther you are from the center of a galaxy.

This means that velocity-related time dilation should be lower for stars farther from the galactic center; it also means that time dilation related to gravitational wells should be lower for these more distant stars.

However, studying the red-shift profiles of galaxies shows that in fact the rotational velocity is fairly constant (and perhaps increasing slightly) with distance from the galactic center. This was one basis for the Dark Matter hypothesis.
https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Taking into account the observed galaxy rotation curve, time dilation related to gravitational wells should be lower for these more distant stars, which is counteracted by velocity-related time dilation which should be roughly constant (or increasing slightly) for stars farther from the galactic center. The overall effect on time dilation would need to be worked out on a case-by-case basis. evan_au, Sat, 23rd Apr 2016

Interestingly, the central stars of planetary systems should also experience lower time dilation than the orbiting planets. jeffreyH, Sun, 24th Apr 2016


This is the opposite of the way I understood it?

My understanding was that the location closest to the bottom of the gravitational well (like the Sun's surface or Mercury) would experience the most gravitational time dilation, while locations farthest from the gravitational well (Pluto, the edge of our galaxy, or the Small Magellenic Cloud) would experience the least time dilation. Earth is somewhere in the middle of this range.

This is confirmed by the fact that the gravitational time dilation is measurable in the precession of the perihelion of Mercury's orbit, but is negligible in the orbits of more distant planets.

Or by "lower time dilation", do you mean "time moves more slowly" (as seen by a distant observer)? evan_au, Sun, 24th Apr 2016


This is the opposite of the way I understood it?

My understanding was that the location closest to the bottom of the gravitational well (like the Sun's surface or Mercury) would experience the most gravitational time dilation, while locations farthest from the gravitational well (Pluto, the edge of our galaxy, or the Small Magellenic Cloud) would experience the least time dilation. Earth is somewhere in the middle of this range.

This is confirmed by the fact that the gravitational time dilation is measurable in the precession of the perihelion of Mercury's orbit, but is negligible in the orbits of more distant planets.

Or by "lower time dilation", do you mean "time moves more slowly" (as seen by a distant observer)?


No I stated a thought process badly. I did not engage brain before typing. I was comparing the path length of a star around the galaxy with those of orbiting planets. If the star described a hypothetical perfectly circular galactic orbit then the path lengths of the planets would be stretched and meandering in comparison. jeffreyH, Sun, 24th Apr 2016

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